A thermal battery is a special purpose dc power source used mainly in military and aerospace applications where a long shelf life, on the order of 10 to 15 years, and a short operating time, on the order of 1 to 5 minutes, is anticipated. It must be compact and sturdy enough to function under high gravity loads and over a wide temperature range. A thermal battery is characterized by an internal pyrotechnic heat source which, upon reaction melts the electrolyte to activate the battery.
The first thermal batteries were developed in Germany during World War II and were intended for use in the V-I and V-II rockets. Progressing through the fifties, the early designs, often referred to as "cup" or "closed cup" systems, were a stack of disc-shaped cell cups which served as the cathode current collector of the cell and contained electrolyte pads and depolarizer pads used in the electro chemical reaction. To provide the heat source, heat pads were inserted between the cell cups. They were typically made by passing paper or a fiberglass tape through a vat containing a slurry of zirconium fuel and a barium chromate oxidizer. The paper was then dried and heat pads punched out. The pyrotechnic pads heat the electrolyte to a 500.degree. to 600.degree. C. operating range but are disadvantageous in that they may change volume during combustion and deform under heavy g-loads experienced in some applications.
In the mid-fifties the then Naval Ordnance Laboratory and the Eureka-Williams Company developed a "pellet" type of cell which, through refinements, has become the present state of the art. In so-called pellet technology the anode and a "DEB pellet", consisting of the depolarizer, electrolyte, and binder, and a heat pellet are pressed together into a single cell for the battery. The heat pellet is typically a pressed disc of powdered iron fuel and potassium perchlorate oxidizer. This pellet meets most of the requirements of thermal battery operation. It ignites easily, rapidly and safely. It also has the necessary conductivity for pellet technology to electrically connect the cells and delivers the necessary heat to maintain the LiCl-KCL electrolyte at the approximate operating temperature of 485.degree. C. Many production batteries, however, exhibit heating problems such as excessive heat (2000.degree. K.) reactions with the electrolyte and migration of fuel from the heat pad or pellet into the electrolyte. Their reaction is also not completely gasless, which can rupture the sealed battery case.
Present day heat sources using iron fuel have the further drawback of requiring at least 150 msec to attain operating temperature. This is an excessive delay for many battery applications, such as fusing circuits in guided missiles.
Another difficulty encountered by both cup and pellet battery heat sources relates to procurement. Barium chromate and most other products such as heat paper or heat pads and including fine (3 .mu.m) zirconium powder are sole source items.